1. Field of the Invention
Embodiments of the present invention relate to a signal transmission and reception system, an installation method of the signal transmission and reception system, and a plant applied with the signal transmission and reception system.
2. Description of the Related Art
In an industrial plant and a power generation plant (hereinafter, simply referred to as “plant”), many instruments, sensors, or the likes (which will be hereinafter referred to as “sensors”) are installed for monitoring and control of the plant. For example, a large-scale plant includes several hundreds of sensors. In above-mentioned plant, signals generated by the sensors are inputted into a monitoring and control device via laid cables. The monitoring and control device uses the signals received from the sensors to monitor and control the plant.
In recent years, from the viewpoint of economic advantage, a technique for transmitting and receiving the signals by “radio” has been proposed in order to reduce and streamline the cables.
A transmitter receives the signal generated by the sensor (or instrument), and transmits the signal after converting the signal into a radio signal. Several hundreds of the transmitters are required according to the number of the sensors, and hence the transmitter needs to be low cost and to be able to be operated by a battery having small capacity. Therefore, the power of the radio signal transmitted from the transmitter is relatively low.
Further, a receiver (antenna) for receiving the signal needs to be installed relatively in the vicinity of the transmitter. This limitation on the distance provides consistency with the radio wave regulation law or a security benefit of preventing the leakage of radio wave to the outside of the plant.
Conventionally, for example, in Patent Document 1 (Japanese Patent Laid-Open No. 2010-124027), a signal transmission and reception technique utilizing a short-distance radio network is proposed.
The sensors are attached in a site area that is a site environment in which facilities or devices of a plant are most intensively arranged. In the site area, facilities or devices, such as a pump, a fan, a turbine, and a valve, and pipes are arranged in the plant. To put it briefly, each of the sensors (and transmitters) is installed in an area surrounded by many obstacles, such as facilities or devices, arranged in the plant.
Therefore, in the case where a short-distance radio network is applied, these obstacles are located between the transmitter and the receiver so as to hinder the transmission of the signal, and thereby cause trouble in transmission and reception of the signal. Further, in the short-distance radio network, the power of the radio signal is low, and hence it cannot be expected that the reflection wave generated by the radio signal being reflected by the obstacles is received by the receiver. This makes the above-described problem more serious.
In the technique proposed in Patent Document 1, in the case where the other facilities or devices around the transmitter and the receiver become obstacles of radio waves, a radio sensor node (transmitter) is reinstalled in a place from which a radio repeater (receiver) can be seen. The technique proposed in Patent Document 1 is effective as a measure against the above-described problem in the case where the number of instruments or sensors is relatively small. However, in the case where several hundreds of instruments or sensors are provided, the technique is not necessarily realistic. For example, it is not easy to reinstall many transmitters and receivers at places where each of the transmitters is within line of sight of one or more of the receivers.